Interaction-selective molecular sieving adsorbent for direct separation of ethylene from senary C2-C4 olefin/paraffin mixture

Olefin/paraffin separations are among the most energy-intensive processes in the petrochemical industry, with ethylene being the most widely consumed chemical feedstock. Adsorptive separation utilizing molecular sieving adsorbents can optimize energy efficiency, whereas the size-exclusive mechanism alone cannot achieve multiple olefin/paraffin sieving in a single adsorbent. Herein, an unprecedented sieving adsorbent, BFFOUR-Cu-dpds (BFFOUR = BF4-, dpds = 4,4’-bipyridinedisulfide), is reported for simultaneous sieving of C2-C4 olefins from their corresponding paraffins. The interlayer spaces can be selectively opened through stronger guest-host interactions induced by unsaturated C = C bonds in olefins, as opposed to saturated paraffins. In equimolar six-component breakthrough experiments (C2H4/C2H6/C3H6/C3H8/n-C4H8/n-C4H10), BFFOUR-Cu-dpds can simultaneously divide olefins from paraffins in the first column, while high-purity ethylene ( > 99.99%) can be directly obtained through the subsequent column using granular porous carbons. Moreover, gas-loaded single-crystal analysis, in-situ infrared spectroscopy measurements, and computational simulations demonstrate the accommodation patterns, interaction bonds, and energy pathways for olefin/paraffin separations.

Comment 3. Related to #2, how did the lattice structure change when calculating the energy landscape?It is surprising that energy barriers for paraffins are so high (several eV).Did authors use an enlarged lattice structure for olefins and the original crystal structure for paraffins?
Author Response: Thank you for the valuable comment.For the calculation of transition states, the activated structure was applied for all calculations with 1×1×2 superlattices.For gas-loaded models, we introduce a molecule into a single lattice by locating the task in the sorption module, then expand the lattice to 1×1×2 superlattices and complete structural relaxation by the Dmol3 module.Such high energy barriers indicate that the paraffin molecules are difficult to diffuse into the frameworks, consistent with the experimental data.
Comment 4. Recent studies also show the similar MOF with anionic pillars (e.g SIFSIX-Cu-TPBDA) are good for acetylene purification.Can BFFOUR-Cu-dpds be used for acetylene purification as well?
Author Response: Thank you for the valuable suggestion.The C2H2 purification (C2H2/CO2) is important in the chemical industry.As recommended, the adsorption isotherms for C2H2 and CO2 were collected at 298 K (Figure R1).The BFFOUR-Cu-dpds adsorbent exhibited a higher C2H2 adsorption capacity (35.6 cm 3 g -1 ) compared to that of CO2 (15.0 cm 3 g -1 ), indicating the application potential for C2H2 purification.The calculated IAST selectivity for C2H2/CO2 (50/50) was 111.4,surpassing many top-ranking MOF adsorbents.For the gas-mixture of C2H4/C2H6 (0.5/0.5, v/v), C2H6 was rapidly eluted from the column at a flow rate of 1.0 ml min -1 , while C2H4 exhibited substantial retention in the column for 28 min until saturation (Figure 5A).
Notably, the efficient C2H4/C2H6 separation was also obtained despite slightly decreased retention time under humid conditions (RH = 61.9%).

Manuscript: Page 19
For the breakthrough experiments with water vapor, the gas mixture passed through a water vapor saturator under ambient conditions.The humidity was measured by a relative humidity meter (UT333, UNI-T, China) at the inlet of the column.Following each breakthrough experiment, the sample was desorbed in-situ in the column with a He sweeping of 5 mL min -1 at 333 K for 2 hours and attained high-purity olefin gas (99.5%).
Reviewer #2 (Remarks to the Author): The authors developed a new 3D Cu-based MOF denoted to BFFOUR-Cu-dpds, enabling to discriminating of the molecules between olefins and paraffin molecules of C2-C4 coupled with high separation performance.The characterization and assessment of separation performance results were well organized and explained logically.This adsorption characteristic is highly promising for separating the olefins from olefins/paraffins gas mixture with an adsorption-based separation process.In addition, this MS will be interested in the relevant MOF and separation research field.Thus, the reviewer recommends the publication of "Nat.Commun.Journal" after addressing minor points as follows: Author Response: We thank Reviewer #2 for the positive and constructive comments.
Comment 1.Why was the stability test examined only in the pH ranges of 5-13?How about the stability at lower pH less than 5? In addition, the used acid and base solvent should be provided more detail in SI.
Author Response: Thank you for the valuable comment.The stability of adsorbents is a critical parameter in industry applications.As recommended, we conducted the stability tests in aqueous solutions with pH=1 and 3.As shown in Figure S13, the intensity of characteristic PXRD peaks significantly decreased, indicating its partial structure decomposition.Detailed information for used acid (hydrochloric acid) and base (sodium hydroxide) solvents with various concentrations has been added in the revised Manuscript.

Structure stability Tests.
To evaluate the solvent stability, 100 mg BFFOUR-Cu-dpds were placed separately in 20 mL vials with 15 mL of different organic and pH solvents for 7 days.The resulting solid was filtered, activated at 80 ℃ for 12 hours, and characterized by PXRD and adsorption analysis.The used acid was hydrochloric acid solution and sodium hydroxide solution with various concentrations.

Comment 2. In the fitting results for C2-C3 paraffins with the DSLF model, the minus values of N2MAX are not acceptable. The authors should use the appropriate fitting model or fitting results to ensure the validity of IAST selectivity values from the model-fitted parameters.
Author Response: Thank you for pointing out the issue.We have re-calculated and confirmed the fitting parameters.The corresponding IAST selectivites have been updated, the C2H4/C2H6 selectivity increased from 28.4 to 68.8, and the C3H6/C3H8 selectivity increased from 103.6 to 108.4.

Modifications:
Supporting information: Page 24 b2 Author Response: Thank you for the valuable comment.The experimental procedure for assessing the kinetic measurement was supplemented in the revised Manuscript.

Kinetic adsorption measurements.
Intelligent Gravimetric Analyzer (IGA-100, HIDEN) was used to measure the time-dependent adsorption profiles of olefins and paraffins.About 80 mg of BFFOUR-Cu-dpds were subjected to evacuation under a high vacuum at 353 K for 24 hours before each adsorption measurement.After being cooled to a specific temperature, a single-component gas or olefin/paraffin gas-mixture was introduced into the chamber, and the mass of the sample loaded with gas molecules was continuously recorded for 80 min.
Comment 4. In the SI, the authors mention the calculation of "static binding energy", like the case of ref. 36.
Please explain the related results in the MS.
Author Response: Thank you for the comment.As recommended, the static binding energy calculated by the DFT method have been supplemented in the revised Manuscript (Page 12).Static binding energy refers to the energy differences before and after adsorption at 0 K, representing the binding strength between adsorbents and adsorbates.The static binding energy was calculated to be 59.43 kJ mol -1 , 71.87 kJ mol -1 , and 82.52 kJ mol -1 for C2H4, C3H6, and n-C4H8, respectively.The calculation method has also been modified in the revised Manuscript.

Modifications:
Manuscript: Page 12 The static binding energy was calculated to be 59.43 kJ mol -1 , 71.87 kJ mol -1 , and 82.52 kJ mol -1 for C2H4, C3H6, and n-C4H8, respectively.The trend of binding strengths was consistent with the Qst values.
Manuscript: Page 21 According to the output files from the CASTEP modules, the "NB dispersion corrected est.0 K energy* (Ecor-0.5TS)" was selected as the energy at 0 K.
Comment 5.The authors mention that the CASTEP module was used to calculate the static binding energy (in the SI), and "energy barrier" was calculated using the CASTEP module in ref. 36.However, in the SI, the authors mention that the DMol3 module was used to calculate the energy barrier.Is there a particular reason why the authors used not CASTEP but DMol3 for energy barrier calculation?And, computational details related to energy barrier calculation in the SI seem to be not for DMol3 but for CASTEP, given the energy unit (eV).Please clarify the computational details.
Author Response: Thank you for pointing out this mistake.We actually use the DMol3 module to calculate the diffusion energy barrier, because DMol3 has a faster calculation speed and relatively high calculation accuracy than CASTEP.The energy unit calculated by DMol3 is Ha (Hatree), and for a clearer representation, we converted it into eV unit according to 1 Ha=27.212eV.

Modifications:
Manuscript: Page 21 The energy barrier calculations were carried out using the DMol3 module in Materials Studio.The periodic slab models with periodic boundary conditions were used to represent BFFOUR-Cu-dpds pore surface.The unit cells were optimized until the force acting between atoms was below 0.002 Ha/Å with SCF convergence of 10 -6 .The Global orbital cutoff was 5.1 Å.The unit conversion followed the equation: 1 Ha=27.212eV.
Comment 6.The authors performed the isotherm and kinetic adsorption measurement with equimolar paraffin/olefin gas mixture to present evidence for the exclusive adsorption of paraffins (Fig. S26-27).
However, these experiments are unsuitable evidence for rejecting paraffin adsorption.To provide direct evidence for adsorption only olefins, the analysis of bulk gas composition should be provided while measuring adsorption for both isotherm and kinetic adsorption.The reviewer thought that providing adsorption and desorption profiles in selected representative conditions was enough to prove the absence of co-adsorption between olefins/paraffins.For this consideration, desorption profiles in Fig S39a-c, C2H6, C3H8, n-C4H10 were checked, and the paraffins (C2-C4) were concurrently detected in the outlet stream during He purges at 333 K. Consequently, the partial co-adsorption of paraffins occurred while measuring the dynamic adsorption of C2-C4 gas mixtures.
Author Response: Thank you for pointing out this issue.We agree that the gas-mixture isotherms cannot fully address the co-adsorption issue.While, some literature also used this as indirect evidence for demonstrating this phenomenon (Chem, 2021(Chem, , 7, 1006(Chem, -1019;;Science, 2016, 353,137-140;J. Am. Chem. Soc. 2020, 142, 41, 17795-17801).Furthermore, we also noticed that the immediate detection of C2-C4 paraffines in the desorption curve in Figures S40a-c, which could be ascribed to the surface adsorption on adsorbents or partial retention in quartz cotton at the inlet and outlet of adsorption column.To demonstrate this issue, we also carried out the breakthrough experiments with He/olefin/paraffin mixture (Figure S41), where He should not be adsorbed.The C2-C4 olefines concurrently eluted with He, suggesting the exclusion of C2-C4 paraffines.

Modifications:
Manuscript: Page 14 In addition, we introduced 20% inert He into olefin/paraffin gas-mixtures (He/olefin/paraffin, 0.2/0.4/0.4,v/v/v) to validate the absence of co-adsorption phenomenon during breakthrough experiments at a flow rate of 2.5 mL min -1 .As shown in Figure S41, the paraffins and He almost concurrently outflow from the column, indicating that the negligible paraffin adsorptions in gas-mixtures similar to He.

Comment 7. The schematic of the breakthrough set-up in Fig S38 was inconsistent with explaining the transient breakthrough experiment in the SI part (MS or GC detector).
Author Response: Thank you for pointing out this mistake.The detector is GC, which has been corrected in Author Response: Thank you for the comment.The dynamic selectivity is also a critical parameter to evaluate the separation performances.The dynamic selectivity was calculated to be 9.16, 8.76, and 3.18 for equimolar C2H4/C2H6, C3H6/C3H8, and n-C4H8/n-C4H10 gas-mixtures, respectively.Due to the different calculation methods and indications, these values are not close to that of IAST predictions.The Ideal Adsorption Solution Theory (IAST) is a theory used to describe the adsorption isotherms of ideal mixed solutions/gases, which assumes that the adsorption of mixed solutions/gases is completely thermodynamic controlled (AIChE J., 1965, 11: 121-127).While, the dynamic selectivity was determined by the adsorption capacity ratio of olefins and alkanes in the purge gas (Angew.Chem.Int. Ed. 2021, 60, 22865-22870).

Manuscript: Page 19-20
Dynamic separation selectivity (α) was obtained by calculating the integral area of the desorption curves of alkanes and olefins during the blowing process.The calculation formula is as follows: Here Si is the integral area of the desorption curve of gas i, t1 and t2 represent the start and end times of desorption (min), F0 and F are the inlet and outlet gas molar flow rates, y1 and y2 represent the mole fractions of 1 and 2. Comment 10.Minor points need to be revised as follows: a) The denotation for "Relative pressure (P/P0)" of the x-axis in all isotherm curves for olefins and paraffins should be revised to Pressure (bar).
Author Response: Thank you for the valuable suggestion.As recommended, we have changed "P/P0" to "bar" in related figures.
Author Response: Thank you for the valuable suggestion.As recommended, we have modified the sentence.
Similarly, the adsorption equilibrium of C3H6 was attained within 9 min in BFFOUR-Cu-dpds and could be easily regenerated under the same conditions within 12 min (Figure 2F).
d) In the legend of Figure S1, the unit of kinetic diameter difference between C2 molecules "0.028 nm Å" needs to be corrected to "0.028 nm."

Fig. S20 :
Fig. S20: Since the transient adsorpfion curves were provided, the diffusional fime constants (D/r2) should be calculated using a proper model to analyze the kinefic data more clearly.

Fig. S29 :
Fig. S29: The trends of the Qst values with gas loadings are different among the three gases.Some explanafions should be provided.